The invention relates to a low-pressure gas discharge lamp comprising a discharge vessel which encloses a discharge space provided with a gas filling in a gastight manner, which discharge vessel comprises tubular end portions each with a capacitive coupling element made of an electrically insulating material for generating and maintaining a discharge in the discharge space.
Gas discharge lamps have until now consisted of a discharge vessel filled with a filling of, for example, mercury and a rare gas in which the discharge takes place, as well as usually two metal electrodes fused into the discharge vessel. One of the electrodes supplies the electrons necessary for the discharge, which electrons are returned to the external current circuit again via the other electrode. The generation of electrons usually takes place through glow emission (hot electrodes), or alternatively through emission in a strong electric field or directly through ion bombardment (cold electrodes). In an inductive mode of operation, the electrons are directly produced in the gas filling across an electromagnetic AC field of high frequency (typically of the order of 1 MHz in the case of low-pressure gas discharge lamps). The electrons then move along closed trajectories inside the discharge vessel, and the usual electrodes are absent. Capacitive coupling elements are used as the electrodes in the case of a capacitive mode of operation. These elements are often formed from electrically insulating materials (xe2x80x9cdielectricsxe2x80x9d) which at one end extend into the discharge vessel and at the other end are connected with electrical conduction to the external current circuit (for example by means of an interposed metal contact). An AC voltage applied to the capacitive electrodes creates an AC electric field in the discharge vessel, with the result that the electrons move along electric field lines of the AC field.
It is a disadvantage of capacitive gas discharge lamps that they generally are found to have a shorter useful life than the gas discharge lamps mentioned further above, in which the generation of electrons is achieved through glow emission.
It is an object of the invention to counteract the disadvantage of the prior art as mentioned above, i.e. to provide a gas discharge lamp of the capacitive type which has a longer operational life than was hitherto usual.
According to the invention, a gas discharge lamp of the kind mentioned in the opening paragraph is for this purpose characterized in that the lamp is provided with means for preventing the occurrence of (hair) cracks in a wall of the discharge vessel as a result of piezoelectric properties of the electrically insulating material. The invention is based on the recognition not previously reached that the operational life of known gas discharge lamps is limited by the fact that the electrically insulating material (xe2x80x9cdielectricxe2x80x9d) of the capacitive coupling elements also has an undesirable side effect, i.e. that it has piezoelectric properties caused by the usually high dielectric constant of the material, which may give rise to vibrations in said material, which in its turn may lead to (hair) cracks in the (glass) wall of the discharge vessel, with all the adverse effects thereof on lamp life. These effects are considerably reduced if the coupling elements after an initial starting phase are heated up to the Curie temperature of the material during switching-on of the lamp.
In a preferred embodiment of a low-pressure gas discharge lamp according to the invention, said means comprise the wall of the discharge vessel, said wall having at least one region of reduced thickness. Said region is formed in particular by a circumferential region, i.e. the region extends along part of the length of the wall of the discharge vessel along the circumference thereof. Providing the wall with a smaller thickness locally, i.e. at the area of said region of the wall, achieves that the wall is heated up more quickly in said area owing to a smaller thermal mass, and accordingly reaches its Curie temperature more quickly. Moreover, said region acts as a vibration damper for the remaining portion of the wall, especially if said region is situated adjacent the joint between the capacitive coupling element and the wall of the discharge vessel.
In a further preferred embodiment of a low-pressure gas discharge lamp according to the invention, said region has a thickness smaller than 0.4 mm. Research has shown that such a thickness for this region prevents the occurrence of said (hair) cracks owing to piezoelectric properties of the electrically insulating material of the coupling elements, while nevertheless a sufficient mechanical strength of the discharge vessel wall is obtained.
In a further preferred embodiment of a low-pressure gas discharge lamp according to the invention, at least substantially the entire wall of the discharge vessel has a thickness smaller than 0.4 mm.
In a further preferred embodiment of a low-pressure gas discharge lamp according to the invention, the capacitive coupling elements have a reduced thickness at their ends facing towards the tubular end portions of the discharge vessel. In another preferred, modified version, the capacitive coupling elements also have a reduced thickness at their ends facing away from the tubular end portions of the discharge vessel. In either case, the wall of the discharge vessel may be connected to the capacitive coupling elements adjacent the thinner ends thereof, so that fewer vibrations are introduced into the wall, while in the latter case also a closing cap connected to the ends facing away from the tubular end portions of the discharge vessel is subjected to vibrations to a lesser degree.